Bob,
I have got better quotes from Trelleborg, but only when I am using a
standard design, but the best I remember was ~$50 per seal at low quantity,
but we did have to modify our design to match a seal they had on the shelf.
Your prices seem about normal for bespoke seal, So I understand the drive
to get the McMaster seals to work, especially for hobby projects where it
is your own money. I hope that keeping the shafts very tight with no
run-out works, it is at least worth a try.
For the Molykote Z i looked at my notes, and the one reference I recalled
for water being an issue was for 10 micron MoS2 and bearing life if reduce
by a factor of 100 in a humid enviorment vs a dry enviorment. But we are
talking the difference between 5k cycles and 100k cycles so probably not a
problem for a quarter turn ball valve. In addition friction coefficient is
0.02 dry and 0.08 wet with MoS2 so it will also reduce torque to keep it
dry. So TL/DR is MoS2 is better that no lube even when wet, but you get
even better performance if you keep the dry lubed parts and dry.
My Ref.
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790004987.pdf
Lloyd
On Wed, May 25, 2016 at 10:16 AM, Robert Watzlavick <rocket@xxxxxxxxxxxxxx>
wrote:
When I talked to the BalSeal rep, he thought the McMaster parts might be
from their company. But since McMaster doesn't list the brand (unlike
Grainger), it may be whoever has the lowest bid. I got a quote from
BalSeal from some custom seals that had a heel you could grip to keep the
OD attached and a different material/seal lip but it was way out of my
price range ($600 NRE plus $100/seal I think). I've had such good luck
with the larger McMaster seals as static seals for my injector so I figured
they might work for a dynamic application. I can say that when I tested
them with LN2, they sealed initially then leaked upon turning so hopefully
if I can keep the shaft from wobbling around I'll have better luck. Do you
have any price quotes for small quantities (20x) of the other brands?
I had planned on testing with water prior and then drying it out without
disassembly prior to the LN2 testing. What does the water do to the
Molykote Z? I soaked the bearings in acetone for a few hours, rotating
them occasionally to get the existing lubricant out of them but I didn't
re-lubricate them. Since the motion is just intermittent, I thought I
would just run them dry. I cleaned the bearings in my LOX flowmeter the
same way and ran them dry.
-Bob
On 05/23/2016 09:46 PM, Lloyd Droppers wrote:
Looks nice Bob, and the bearing arrangement should work out. I think the
Belleville washer that other people suggested might be nice, but if
everything in the load path (shaft, bearings, nut and spacers) have the
same CTE the belleville washer shouldn't be necessary.
My main concern about the valve, if I were building it, would be the PTFE
spring energized seals. I have never been able to get the cheap mcmaster
versions (the used to be Coors-Tek brand) to work without leaking even with
good surface finishes. So if they end up leaking on you, I would recommend
spending a bit more on something from BAL or Trelleborg before you throw
the design out as not working as I have had much more luck with the higher
quality shaft seals.
For the lubrication you shouldn't certainly shouldn't use krytox, it will
freeze at cryo temperatures. The Molykote Z works fine if you keep it dry.
Speaking of dry make sure to purge the cavity with dry gas as ball bearings
really do not like ice buildup, but you have done this before and probably
knew that already.
Lloyd
